Synthesis gases are conventionally produced by conducting an endothermic steam methane reforming reaction within a steam methane reformer. A steam methane reformer has a radiant section containing catalyst filled tubes to react a hydrocarbon containing reactant and steam and a convective section in which heat is recovered from the flue gas for such purposes as generating the steam. The heat required to support the endothermic reforming reaction is produced by burners that project flames into the radiant section of the reformer.
There are advantages to generating synthesis gases in a combined reforming process that utilize a primary reforming stage that can be formed of a conventional fired steam methane reformer, as described above, and a secondary reforming stage that can incorporate an oxygen transport membrane reactor to react residual methane with oxygen separated from an oxygen containing gas through oxygen ion transport. Such a system is described in U.S. Pat. No. 6,110,979. In this patent, a steam and hydrocarbon containing feed is heated and introduced into a steam methane reformer. The partially reformed feed is then introduced into an oxygen transport membrane reactor to produce a synthesis gas stream that is further processed to recover hydrogen.
The problem with typical combined reforming systems, including such systems that utilize an oxygen transport membrane reactor, is that the carbon dioxide produced by the requisite firing to generate the heat necessary to support the endothermic reforming reaction is simply discharged to the environment as stack gases. In addition to the possible environmental problems presented by such operation, the carbon dioxide itself is a valuable product that can be used in a variety of products and industrial processes, for example, carbon dioxide enhanced oil recovery.
It is to be noted that it has been suggested in the art to use an oxygen transport membrane, such as illustrated in U.S. Pat. No. 5,888,272, to generate a heated combustion gas stream to supply heat to a downstream process that requires heat. Oxygen transport membrane separate oxygen from the oxygen containing feed through use of a ceramic material that at elevated temperatures will conduct oxygen ions. When a driving force such as a partial pressure differential is applied to such a material, oxygen will ionize at one surface of the material or more properly, membrane. The oxygen ions are transported through the membrane and emerge at the other side to recombine into elemental oxygen. As a result of such recombination, electrons are transported back through the membrane to ionize the oxygen. The oxygen partial pressure differential can be produced by combusting a fuel at the surface of the membrane at which the oxygen ions emerge and recombine.
The problem with utilizing an oxygen transport membrane to generate heated flue gases to supply heat to a steam methane reformer is that the burners normally used operate at adiabatic flame temperatures in excess of 1500° C. An oxygen transport membrane, however, cannot tolerate such temperature for an extended period of time without eventually suffering structural failure. A typical operational temperature range of an ion transport membrane is between about 400° C. and about 1200° C. Therefore, the use of an oxygen transport membrane for purposes that involve the heating of a steam methane reformer has proven to be impractical.
As will be discussed, the present invention provides a method in which an oxygen transport membrane combustion device is utilized to combust a fuel and generate a flue gas to supply heat to a steam methane reformer and to in turn allow carbon dioxide produced by the combustion to be easily separated and that does not require high, impractical operational temperatures for the oxygen transport membrane.